The present invention relates to a diffusion-limiting membrane holding means for a sensor, and more particularly to a diffusion-limiting membrane holding means for a sensor suitably used for sticking a diffusion-limiting membrane to the surface of an enzyme electrode unit of a sensor.
It is known that a physiologic active substance has a characteristic capable of selectively detecting a very complicated organic compound, protein or the like with high sensitivity. With attention directed to this characteristic, researches and developments have been made on measurement of such organic compound, protein or the like with the use of an enzyme electrode unit having base electrodes on which a physiologic active substance is immobilized.
When measuring a target substance with the use of the enzyme electrode unit above-mentioned, the target substance is oxidized or reduced under the presence of such physiologic active substance. The concentration of the target substance is determined by measuring the amount of a substance produced or consumed in such oxidation or reduction. Accordingly, the upper limit of concentration which can be measured, is determined dependent on the amount of a substance provoking such oxidation or reduction, for example the amount of oxygen.
In view of the foregoing, it has been proposed to increase the concentration measuring limit by limiting the penetration rate of a target substance by a diffusion-limiting membrane mounted on the surface of an enzyme-immobilized membrane in/on which a physiologic active substance is immobilized.
More specifically, there has been adopted an arrangement in which the diffusion-limiting membrane is mounted on a cap to be threadedly secured to the base portion of a rod-like enzyme electrode unit, and screwing the cap causes the diffusion-limiting membrane to be automatically stuck to the enzyme-immobilized membrane.
With the use of such arrangement, the penetration rate of a target substance to be measured is limited by the diffusion-limiting membrane, thereby to achieve measurement of a considerably high concentration. To eliminate the influence of interfering substances contained in a target solution to be measured (for example, increase in diffusion limiting effect resulting from the sticking of such interfering substances), the diffusion-limiting membrane needs to be replaceable. This is the reason of why the membrane screwing mechanism by a cap is adopted.
When the diffusion-limiting membrane holding means having the arrangement above-mentioned is used, replacement of the diffusion-limiting membrane may be relatively facilitated. There are instances, however, where it becomes very difficult to mount or remove the holding means on or from the base portion of an enzyme electrode unit due to the arrangement of its mounting mechanism, or where it is not possible to achieve a uniform adhesion of the diffusion-limiting membrane to the enzyme-immobilized membrane due to the degree of the screwing force.
Further, when the enzyme electrode unit base portion has a small diameter, resulting in decrease in the cap size, this causes the manual mounting/removal operation to be very difficult. This makes the problems above-mentioned more conspicuous.
Moreover, the diffusion-limiting membrane is mounted on a cap, requiring a large space for preserving and/or transportiong the same.
Further, the enzyme electrode unit has a convex curved surface having a predetermined radius of curvature, while the diffusion-limiting membrane mounting surface of the cap is made in a plane surface having neither convex nor concave portions. Accordingly, the diffusion-limiting membrane is forcibly press-contacted with the surface of the enzyme electrode unit by force exerted on only the edge of an opening through which a target solution to be measured penetrates. Therefore, the diffusion-limiting membrane or the enzyme-immobilized membrane secured to the surface of the enzyme electrode unit is susceptible to damages.
A pressing force of the diffusion-limiting membrane to the enzyme-immobilized membrane is exerted only in points or in a line. This assures no uniform contact therebetween throughout the surfaces. Accordingly, when the diffusion-limiting membrane is replaced, measured data may vary to deteriorate the reproducibility.
Further, a target solution to be measured is guided to the diffusion-limiting membrane through a relatively small opening formed in the diffusion-limiting membrane holding means. This requires to accurately set the dropping position of the target solution, decreasing the operational efficiency.
More specifically, the diffusion-limiting membrane holding means may be hydrophilic or hydrophobic dependent on the material thereof.
When the holding means is hydrophilic, not only that surface of the diffusion-limiting membrane holding means on which a target solution to be measured is dropped, but also the wall of the opening through which the target solution to be measured penetrates, are hydrophilic. If the amount of a target solution to be measured is small, a major portion of the target solution is not guided to the opening but diffuses on the top surface of the diffusion-limiting membrane holding means. This decreases the utility efficiency of the target solution to be measured.
On the contrary, when the holding means is hydrophobic, a target solution to be measured is apt to be repelled by the diffusion-limiting membrane holding means. Accordingly, there is considerably reduced the possibility that the target solution is smoothly guided to the diffusion-limiting membrane through a relatively small opening.
Prior to actual concentration measurement of a target substance, a necessary amount of a standard solution for calibration contained in a vessel is dropped on the diffusion-limiting membrane to obtain a detection signal, based on which an initial calibration operation is then made. In particular, to facilitate the dropping of the standard solution for calibration onto the diffusion-limiting membrane, it may be proposed that the standard solution for calibration contained in the vessel is dropped by pressing the vessel body.
Thus, there are required a target substance concentration measuring apparatus and a vessel for containing a standard solution for calibration, making the measurement very expensive in its entirety. Further, various germs may stick to the vessel to change the concentration of the standard solution for calibration. More specifically, the vessel for containing a standard solution for calibration generally contains a much greater amount of the standard solution or calibration than an amount required for one initial calibration operation. This makes the vessel considerably bulky. Further, with a standard solution for calibration contained therein, the vessel needs to be preserved for a relatively long period of time. If various germs stick to the vessel in such preservation period, substances in the standard solution may be consumed by such various germs. Accordingly, the target substance in the standard solution may be decreased in concentration with the passage of time. This makes it impossible to use such solution as a standard solution for calibration. To prevent the occurrence of such problems, it is required not only to suitably arrange the vessel preserving environment, but also to use care such that the vessel does not come in contact with the cap or the like when the standard solution is dropped. This remarkably decreases the operational efficiency.